MakeUseOf.com has an instructive tutorial on creating a custom background image for those who have their own YouTube channel. If you're trying to draw viewers to your videos content is king, but it helps to have nice design.

Click the link below to get started. Note that background you create will work for both the original YouTube interface as well as the new Cosmic Panda layout. The tutorial assumes you are using Photoshop Elements, but the instructions are generic enough that anyone using a different image editor that has layering capabilities (sorry MS Paint!) should be able to keep up.